The invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle. The invention also concerns an automotive vehicle, such as a truck, equipped with such a valve actuation mechanism.
Automotive vehicles, such as trucks, often rely on an engine brake function to slow down in order, for example, to reduce wear on brake pads, particularly on downward slopes. It is known to perform engine brake by acting on the amount of gas present in the cylinders of the engine in two distinct phases. In a first phase, when the pistons are at near a bottom dead center position, one injects exhaust gases into the chambers of the cylinders so as to slow down the pistons when they move towards their high level. This is done by slightly opening at least a valve connected to an exhaust manifold, while exhaust gases are prevented to be expelled from the exhaust pipe, and thereby at a certain pressure above atmospheric pressure. In the second phase, the gases which are compressed by the piston are expelled from the chamber of the cylinder when the piston is at high level in order to prevent an acceleration of the piston under effect of volumic expansion of compressed gas. This is done by slightly opening a valve so as to expel gases from the cylinder. In most cases, the valve (or valves) which is (are) opened for the engine brake function is (are) a main exhaust valve. An engine brake system is described in document WO 9009514.
To perform these engine brake valves movements, also called engine brake valves lifts, the engine comprises, for each cylinder, a rocker acting on the valves to open and close them. The rocker is acted upon by a rotating cam which has at least one lift sector to cause the lifting (opening) of the valve. If the valve is also an exhaust or an intake valve, the corresponding cam will comprise a main valve lift sector and one or several auxiliary valve lift sectors (also called main valve lift bump and auxiliary valve lift bump) When engine brake is needed, a cam follower surface of the rocker is moved in close contact with a cam of a camshaft moving the rocker so that the brake movements of the valve are obtained, when the cam follower interacts with the auxiliary valve lift sectors. In normal operating conditions of the engine, the valves should not perform these movements and
the roller of the rocker is kept slightly remote from the engine brake rams so that the cam follower does not interact with the auxiliary valve lift sectors. The distance or clearance between the roller and the engine brake cams ensures that only the larger main lift sector on the cam, dedicated to the main exhaust event, causes an opening of the exhaust valve, but not one or several smaller auxiliary lift sectors dedicated to the engine brake function. This clearance is suppressed when engine brake is needed, by moving an activation piston of the rocker to make to close contact between the roller and the cam, so that engine brake dedicated lift sectors on the cam also cause an opening of the valve. An engine brake system having such valve actuation mechanism is described in WO-91/08381
In the case of a system where two valves are to be actuated, the piston can be in contact with the valves through a valve bridge.
When the engine brake valve opening(s) have been performed, a reset function is preferably to be performed. In other words, the activation piston needs to be moved towards its initial position in order to ensure that the valves are closed early enough in order to extended valve lift overlap which could compromise engine brake performance.
Engine brake systems generally comprise a control valve to direct pressurized control fluid pressure in a chamber adjacent to the piston to move the piston from its initial position to its engine brake actuation position. The control valve controls whether or not the engine brake function is activated. This control valve lets pressurized control fluid flow, at a pressure of for example 2 to 5 bars, towards each rocker as long as the engine brake function is needed, which typically lasts several seconds or tens of seconds during which the engine and the cam shaft may perform several hundreds or thousands of complete revolutions. In some systems, a check valve is provided to prevent any fluid flow out of the chamber. In some known systems, such as the one described in WO-91/08381, the check valve can nevertheless be forced to an open position, allowing the control fluid to escape the chamber when the engine brake is not needed. This is achieved when no control pressure is sent to the control valve. In known systems, there is only one control valve for several cylinders, so that it is not possible to use the control valve to empty the chamber to allow retraction of the piston, if such retraction is needed for a period of time inferior to one revolution of the camshaft.
It is known, for example from U.S. Pat. No. 6,253,730, to act on the check valve thanks to a stopper which is fixed to a housing of the engine, so as to open the check valve and release fluid pressure in the chamber so that the piston may move towards its initial position, retracted. This technical solution does not insure a satisfying reliability.
The aim of the invention is to provide a new valve actuation mechanism in which, when a specific operation function of the engine must be activated, the activation piston can be reset to its first position in a more efficient and reliable way than in the prior art.
To this end, the invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle, comprising rockers moved by a camshaft, each rocker being adapted to exert a valve opening force on at least a portion of a valve opening actuator of each cylinder belonging to a housing of the engine, via an activation piston, housed in a bore of the rocker, and movable with respect to the rocker under action of a fluid pressure raise in a chamber fluidly linked to the bore, from a first position to a second position, in which a cam follower of the rocker reads at least one auxiliary valve lift sector of a cam of the camshaft so as to perform an engine operating function, each rocker comprising a discharge valve movable in translation with respect to the rocker and adapted to reduce fluid pressure in the chamber. This valve actuation mechanism is characterized in that it comprises, for each rocker, a stopper fast with the housing and adapted to exert, on a portion of the piston, a force for opening the discharge valve when the piston has to be moved from its second position to its first position.
Thanks to the invention, the activation piston is set back to its first position by reducing the fluid pressure in the chamber thanks to the discharge valve, and by a mechanical part acting on the piston independently for each rocker. This improves the reliability of the valve actuation mechanism.
According to further aspects of the invention which are advantageous but not compulsory, such a valve actuation mechanism may incorporate one or several of the following features:
The stopper comprises a rod having an end fixed to the engine by a fastening flange at another end comprising a pushing zone.
The discharge valve is a normally closed valve which is opened by the fluid pressure in the chamber when such pressure exceeds a predetermined threshold.
The discharge valve comprises a plug member, which is spring-biased against a seat realized on a surface of the rocker and which is exposed to a fluid pressure force exerted by fluid in the chamber, said fluid pressure acting on the plug member against the biasing action of the spring.
The discharge valve is carried by the piston.
The discharge valve comprises a ball which is spring biased by a compression spring against a seat extending around a passageway fluidly linking a hollow portion of the piston and the chamber, and wherein the piston comprises at least one bleed passage adapted to allow passage of fluid from the hollow portion of the piston to the outside of the piston.
The force exerted by said compression spring on the ball of the discharge valve is inferior to the force (Fp) exerted by the fluid of the chamber on said ball when the force of the stopper is exerted on the piston.
The piston comprises a first element housed in the bore and movable in translation with respect to the rocker, and a valve member, carrying the discharge valve, housed in a hollow portion of the first element the valve member being movable in translation with respect to the first element along a longitudinal axis of the piston, wherein the valve member comprises a pushing surface adapted to exert the valve opening force on the valve opening actuator, and wherein the valve member comprises at least one bleed passage adapted to allow passage of fluid from the hollow portion of the first element to the outside of the piston.
The valve member is movable with respect to the first element between a first position corresponding to a closed position of the discharge valve, in which a plate of the valve member is maintained, by action of a spring exerting a compression force between the first element and the valve member, in abutment against a stop of the first element, and in which passage of fluid between the chamber and the hollow portion of the first element is blocked, and a second position corresponding to an opened position of the discharge valve, in which the valve member is offset with respect to the first element and fluid can circulate between the chamber and the hollow portion of the first element.
The force of the stopper is exerted by contact between the stopper and the valve member, and wherein the compression force exerted by said spring is inferior to the force exerted by the stopper on the valve member.
The discharge valve is housed in the rocker apart from the piston.
The force of the stopper is exerted by contact between the stopper and the piston, and wherein the force exerted by the fluid in the chamber when the stopper is in contact with the piston is superior to the compression force exerted by said spring.
The rocker comprises a bleed passage fluidly linking the chamber and the outside of the rocker, and adapted to be selectively opened or closed by the discharge valve.
The force exerted by the stopper is variable and becomes superior to a force which keeps the discharge valve in a closed position only when the piston has to be moved from its second position to its first position and remains superior to said force until the piston is back in its first position.
The stopper comprises a main spring adapted, when deformed, to exert a compression force on a pushrod on which the pushing zone is realized.
The valve actuation mechanism is an exhaust valve actuation mechanism.
The activation piston activates an exhaust gases recirculation function when it is in its second position.
The activation piston activates an engine brake function when it is in its second position.
The valve actuation mechanism is an intake valve actuation mechanism.
The invention also concerns an automotive vehicle, such as a truck, comprising a valve actuation mechanism as mentioned here above.